1. Field of the Invention
The invention pertains to a process for preparing organopolysiloxane/polyurea/polyurethane block copolymers, and to the use of the organopolysiloxane/polyurea/polyurethane block copolymers prepared by means of the process.
2. Background Art
The properties of polyurethanes and silicone elastomers are complementary in many areas. Polyurethanes are notable for their outstanding mechanical strength, elasticity, and very good adhesion, abrasion resistance, and ease of processing by extrusion from the melt. Silicone elastomers, on the other hand, possess an excellent temperature stability, UV stability, and weathering stability. They retain their elastic properties at relatively low temperatures and consequently do not tend toward embrittlement. In addition they possess special water repellency and antistick (“release”) surface properties.
Conventional polysiloxanes are employed for elastomers, seals, adhesives, sealants, and antistick coatings in the form of thixotropic pastes. In order to achieve a desired ultimate strength, different methods of curing the compositions have been developed, with the objectives of creating the desired structures with their associated mechanical properties. In the majority of cases, however, the polymers must be blended, for example by the addition of reinforcing additives such as pyrogenic silicas, in order to attain adequate mechanical properties. In curable systems, a distinction is generally made between high temperature vulcanizing (HTV) systems and room temperature vulcanizing (RTV) systems. In the case of the RTV compositions there are both one-component (“RTV-1”) and two-component (“RTV-2”) systems. In the RTV-2 systems, two components are mixed and hence catalytically activated and cured. Several different catalysts and curing mechanisms are available. Curing is normally accomplished by peroxidic crosslinking, by hydrosilylation by means of platinum catalysis, or by condensation reactions. Although such RTV-2 systems possess very long pot lives, the attainment of optimum properties requires very precise compliance with the mixing proportions of the two components, leading to increased complexity of the processing apparatus. RTV-1 systems likewise may cure by peroxidic crosslinking, by hydrosilylation by means of platinum catalysis, or by condensation. In this case, however, either an additional processing step for adding and compounding the crosslinking catalyst is necessary, or the compositions have only a limited pot life. A feature common to all these systems, however, is that the products are insoluble after processing and also cannot be recycled.
Consequently, the combination of urethane polymers and silicone polymers ought to provide access to materials having good mechanical properties, yet which at the same time feature processing possibilities which are greatly simplified as compared with the silicones, while continuing to possess the positive properties of the silicones. The combination of the advantages of both systems may therefore lead to compounds having low glass transition temperatures, low surface energies, improved thermal and photochemical stabilities, low water absorption, and physiological inertness. However, polyurethanes and silicones have limited compatibility, and often segregate into separate phases which together do not offer the desired improved characteristics.
Investigations have been carried out in order to overcome the poor phase compatibilities of the two systems. Adequate compatibilities were achievable in only a few special cases through the production of polymer blends. Not until the preparation of polydiorganosiloxane-urea block copolymers, described in I. Yilgör, POLYMER, 1984 (25), p. 1800, and in EP-A-250248, was it possible to achieve this objective. The reaction of the polymer building blocks takes place ultimately by a comparatively simple polyaddition, similar to that employed for the preparation of polyurethanes and polyureas. As starting materials, bisaminoalkyl-terminated polysiloxanes are used as siloxane building blocks for the siloxane-urea copolymers. They form the soft segments in the copolymers, analogous to the polyethers in pure polyurethane systems. As hard segments use is made of customary diisocyanates, which can also be modified by adding diamines such as 1,6-diaminohexane, or dihydroxy compounds such as butanediol, in order to achieve higher strengths. The reaction of the bisamino compounds with isocyanates is spontaneous and generally requires no catalyst.
The silicone building blocks and isocyanate building blocks of the polymer are readily miscible within a wide range. The mechanical properties are determined by the ratio of the different polymer blocks, e.g. the soft silicone segments and the hard urea segments, and, critically, by the diisocyanate used. As a result of the strong interactions of the hydrogen bonds between the urea units these compounds possess a defined softening point, and the materials obtained are thermoplastic.
Both in Yilgör et al. and in EP-0 250 248 A2 the bisaminoalkyl-functional siloxanes used as starting material are prepared by way of equilibration reactions. EP 0 250 248 A2 describes bisaminoalkyl-terminated polydimethylsiloxane chains which even in relatively high molecular weight ranges, possess sufficient purity to ensure the high molecular weights that are required for good mechanical properties in the end polymers in the reaction with diiosocyanates. These bisaminoalkyl-terminated silicones are prepared exclusively by way of an equilibration reaction carried out in a critical manner employing specific equilibration catalysts.
Difunctional silicone oils prepared by way of equilibration reactions, however, have a number of disadvantages. The equilibration reaction described in EP 0 250 248 A2 is a very protracted reaction. Additionally, it is necessary to use a very expensive starting material such as bisaminopropyltetramethyldisiloxane, together with specific catalysts, which must be synthesized in an extra step. These requirements make the process unfavorable from an economic standpoint. Furthermore, at the end of the equilibration reaction, the catalyst must be either thermally deactivated or neutralized, leaving catalyst residues and hence impurities in the end product, with adverse consequences for the thermal stability of the materials thus produced. These impurities are likewise responsible for a strong intrinsic odor in the materials synthesized from components containing them. It is also necessary to remove about 15% of cyclic siloxanes. From a technical standpoint, however, it is generally not possible to achieve complete removal, and thus cyclic siloxanes remain in the product and may exude from downstream products. In the course of thermal treatment the silicones prepared in this way exhibit a tendency to take on a clearly visible yellow tinge.
From DE 101 37 855 A1 it is known to prepare bisaminoalkyl-terminated siloxanes, which may be used to prepare siloxane-urea copolymers, by reacting hydroxyl-terminated siloxanes with special reactive aminosilanes. This gives materials of heightened purity which are distinguished by a simpilifed preparation process and by good mechanical and optical properties. A disadvantage here again, however, is the separate preparation of a bisaminosilicone from separate starting materials. This bisaminosilicone may easily be contaminated, may gel as a result of formation of carbamates, or may yellow under the influence of oxygen.
It would be desirable to provide a process which yields contamination-free siloxane-urea copolymers which have high molecular weights and, consequently, favorable mechanical properties such as high tensile strength and elongation at break, and which exhibit good processing properties such as low viscosity and absence of solvents. Additionally the process ought ideally to be achievable without substantial technical expenditure, and the materials ought to be preparable from readily available starting materials, in order to be able to compete economically with existing systems. By dispensing with intermediates, technical expenditure can be minimized, and the possibility of contamination of the product and of deviations from process parameters can be reduced as well.